Fortran/OpenMP: Fix mapping of array descriptors and deferred-length strings

Fortran/OpenMP: Fix mapping of array descriptors and deferred-length strings

Previously, array descriptors might have been mapped as 'alloc'
instead of 'to' for 'alloc', not updating the array bounds. The
'alloc' could also appear for 'data exit', failing with a libgomp
assert. In some cases, either array descriptors or deferred-length
string's length variable was not mapped. And, finally, some offset
calculations with array-sections mappings went wrong.

The testcases contain some comment-out tests which require follow-up
work and for which PR exist. Those mostly relate to deferred-length
strings which have several issues beyong OpenMP support.

This is the OG12 variant of the submitted but unreviewed GCC 13/mainline
patch at https://gcc.gnu.org/pipermail/gcc-patches/2023-February/612387.html

gcc/fortran/ChangeLog:

	* trans-decl.cc (gfc_get_symbol_decl): Add attributes
	such as 'declare target' also to hidden artificial
	variable for deferred-length character variables.
	* trans-openmp.cc (gfc_trans_omp_array_section,
	gfc_trans_omp_clauses, gfc_trans_omp_target_exit_data):
	Improve mapping of array descriptors and deferred-length
	string variables.

gcc/ChangeLog:

	* gimplify.cc (gimplify_scan_omp_clauses): Remove Fortran
	special case.

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/target-enter-data-3.f90: Uncomment
	'target exit data'.
	* testsuite/libgomp.fortran/target-enter-data-4.f90: New test.
	* testsuite/libgomp.fortran/target-enter-data-5.f90: New test.
	* testsuite/libgomp.fortran/target-enter-data-6.f90: New test.
	* testsuite/libgomp.fortran/target-enter-data-7.f90: New test.